Functionality to handle user mobility is a fundamental component in wireless communication systems such as cellular networks. From a service quality perspective, such functionality must ensure that service continuity is maintained as user equipments move from one cell to another during an active session, and that each new session is established in a sufficiently good radio environment. From a spectral efficiency perspective, such functionality should ensure that an active user is always served by the most appropriate base station or base stations, which typically may be the closest base station or base stations, and/or the base station or base stations that provide the best radio propagation conditions, at a particular moment.
The 3rd Generation Partnership Program (3GPP) is currently standardizing a new radio access system called the Long Term evolution (LTE) of 3G. Just as some preceding systems, this system will include radio protocols intended to minimize the loss of data over the radio link. The protocol stack includes mechanisms to ensure lossless handover (HO) functionality, and to be specific, the Packet Data Convergence Protocol (PDCP) and the Radio Link Control (RLC) protocols are used for this purpose.
To enable lossless handover functionality, the Radio Bearer (RB) is configured to use the RLC protocol in Acknowledged Mode (AM), and the corresponding PDCP mode for Data RBs (DRB). Each PDCP Protocol Data Unit (PDU) is given to RLC for transmission. The RLC AM functions so that each RLC PDU sent out must be acknowledged (ACKed) by the receiver side to the transmitter side. Acknowledgements (ACKs) are sent upon a POLL request from the transmitter i.e. the serving radio node, and upon T_reOrdering timer expiry at the receiver side, in case the T_Status_Prohibit timer is not running. If the T_Status_Prohibit is running, the RLC STATUS is sent upon the expiry of the timer. The RLC entity receives/delivers RLC Service Data Units (SDU) from/to upper layer and sends/receives RLC PDUs to/from its peer RLC entity via lower layers. One RLC PDU can contain a various number of PDCP PDUs, or segments thereof, according to the instantaneous link bitrate. Upon reception of an ACK for a whole RLC SDU, the RLC protocol must indicate the successful delivery of the higher layer PDU to the higher layer, i.e., to the PDCP transmitter. To ensure a lossless handover, the PDCP transmitter should not discard SDUs from it's buffer before it has received the indication of successful delivery from the RLC transmitter.
Below the RLC layer, a Hybrid ARQ (HARQ) protocol at the MAC level is responsible for the transmission and retransmission of the packets. The HARQ is usually configured so that there is a high probability of the data transmitted from the source radio network node such as e.g. a source eNB, to be successfully received by the User Equipment (UE) without involving retransmissions.
The source radio network node is the node which at the moment is serving the user equipment. The target radio network node, or target eNB, is the node into which the handover is to be performed as the user equipment is moving into the geographical area covered by that radio network node.
During the handover, all the PDCP SDUs that have not been acknowledged by the lower layer are forwarded to the target radio network node from the source radio network node.
The source radio network node may start the data forwarding after it receives handover request acknowledgement from the target radio network node. The user equipment moves to the target radio network node after receiving the handover command from the source radio network node. When in the target cell, the user equipment sends handover confirmation message to the target radio network node. At this stage, the user equipment may also transmit a PDCP Status report if so configured. After reception of the PDCP Status report, the target radio network node may discard the PDCP SDUs that have already been received by the user equipment in the source cell. If the user equipment is not configured to transmit the PDCP Status report to the target radio network node, the target radio network node will transmit all the forwarded SDUs.
A problem with the existing data forwarding mechanism is that the source radio network node is unaware of the latest information of what data the user equipment has received and what it has not received.
During the handover, all the unacknowledged PDCP SDUs are forwarded to the target radio network node by the source radio network node via X2 interface. Many PDCP SDUs that are forwarded to the target radio network node have already been successfully received at the user equipment, and are either dropped at the target radio network node or at the user equipment. Thus, the problem is the inefficient data forwarding mechanism between the source radio network node and target radio network node, which unnecessarily increases the load on the data link X2 connecting the source radio network node and target radio network node.
Since the RLC ACK procedure depends on the POLL and T_reOrdering settings, it could be that many of those SDUs have already been successfully delivered to the user equipment, and would not need to be forwarded to the target radio network node.
A first proposed solution is that the source radio network node polls the user equipment once just after receiving the MEASUREMENT_REPORT from the user equipment. Another proposed solution is to send the RLC status report along with the MEASUREMENT_REPORT in the uplink. The latter solution however requires changes in the 3GPP LTE Release specification 36.331 and 36.322.
Another solution is to send a PDCP Status instead of RLC STATUS message to the source radio network node before the user equipment detaches from the source. This solution requires changes to the 3GPP LTE Release specification 36.331 and 36.323, and has so far not been accepted to be part of the specification.